This invention relates to processes for the conversion of aromatic hydrocarbons, and is more specifically an improved process for disproportionation and transalkylation of aromatic hydrocarbons to obtain xylenes.
The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is paraxylene, the principal feedstock for polyester which continues to enjoy a high growth rate from large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Metaxylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C.sub.8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Most commonly, toluene is dealkylated to produce benzene or disproportionated to yield benzene and C.sub.8 aromatics from which the individual xylene isomers are recovered. More recently, processes have been introduced to disproportionate toluene selectively to obtain higher-than-equilibrium yields of paraxylene.
A current objective of many aromatics complexes is to increase the yield of xylenes and to deemphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. Benzene produced from disproportionation processes often is not sufficiently pure to be competitive in the market. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C.sub.9 aromatics along with toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,016,219 (Kaeding) discloses a process for toluene disproportionation using a catalyst comprising a zeolite which has been modified by the addition of phosphorus in an amount of at least 0.5 mass-%. The crystals of the zeolite are contacted with a phosphorus compound to effect reaction of the zeolite and phosphorus compound. The modified zeolite then may be incorporated into indicated matrix materials.
U.S. Pat. No. 4,097,543 (Haag et aL) teaches toluene disproportionation for the selective production of paraxylene using a zeolite which has undergone controlled precoking. The zeolite may be ion-exchanged with a variety of elements from Group IB to VIII, and composited with a variety of clays and other porous matrix materials.
U.S. Pat. No. 4,629,717 (Chao) discloses a phosphorus-modified alumina hydrogel formed by gelation of a homogeneous hydrosol. The composite has a relatively high surface area of 140-450 m.sup.2 /g and high activity and selectivity in 1-heptene conversion tests.
U.S. Pat. No. 5,169,812 (Kocal et aL) teaches a catalyst for aromatization of light hydrocarbons comprising a zeolite, preferably ZSM-5, a gallium component and an aluminum phosphate binder. The composite is treated with a weakly acidic solution, dried and calcined to increase its tolerance to hydrogen at high temperatures.
Workers in the field of aromatics disproportionation continue to seek processes and catalysts having exceptionally high selectivity for paraxylene from toluene combined with favorable activity and stability.